aristurtle said:
Now suppose for simplicity that the galaxy,and that its physical diameter was w at the time it emitted the light.find the apparent angular size of the galaxy as it would be observed from Earth today.
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Did Wallace's hint---to look up "angular size distance" in your text---work for you or are you still confused?
If you are still confused, you might be more explicit about your situation. You referred to Mr Guth, is that your teacher or is he the author of the book you are studying from like for example Inflationary Universe by Alan Guth:
I can't tell exactly what is puzzling you because you don't say enough.
I can GUESS that what is puzzling you is a kind of wellknown paradox about angular size. Somebody---like your textbook author or your teaching assistant---may have been trying to explain that and gotten you confused.
It does not have to do with galaxies changing their actual physical size. If all galaxies were forever and always the exact same diameter you would still see a curious effect due to the expansion history.
Suppose that all galaxies are and have always been exactly 100,000 LY in diameter.
Then you intuitively expect that the ones that are farther away will "look smaller" in the sense of making a smaller angle in the sky. Like a distant car on the road looks smaller than a nearby car----makes a smaller angle in your field of vison.
But because the universe has expanded it doesn't work like what you intuitively expect.
what we see as the angular size of a galaxy is the angular size that it HAD when it emitted the light
which could be larger than you expect (if it emitted the light a long time ago when it was much closer).
Here is an example. The CMB radiation that people love to make temperature maps of these days is coming to us from hydrogen gas that was once (at the time it emitted the light) 40 million LY from here. Imagine an hot blob of that gas that was diameter 1 million LY. So across the blob makes an angle of 1/40 of a radian. The lightrays coming to us span an angle of 1/40 radian.
Assume the blob stays the same diameter (1 million LY) as it might if it condenses into a gravitationally bound cluster. Stable objects like clusters and galaxies don't participate in the general expansion.
So now that hydrogen is 44 billion LY away from here, because of the 1100-fold expansion which has occurred since the CMB light was emitted. That expansion is why the light took so long to get here, almost the whole 13-some billion years.
Intuitively you think that the angular diameter should be 1/44 MILLIradians. Because the object is one million LY diameter and it is now 44 billion LY away. But the measured angular diameter is not that small. It is actually 1100-fold larger than you expect, namely 1/40 of a radian.
The example illustrates a general fact that stuff at cosmological distances looks larger (angular size) than you'd expect.
That MAY be what the book or the teacher was trying to get across. If it was something else and you are still puzzled, keep asking and make it more clear what the problem is.